Concave cross-grating spectrometer: a reduction to the necessary minimum

This contribution presents the concept, optical design, implementation, and testing of a compact cross-grating spectrometer that integrates all optical functionalities of a classical Echelle spectrometer, including imaging and two-dimensional dispersion, into a single optical element. This element is a concave cross-grating formed by the superposition of two perpendicularly oriented blazed gratings on a concave surface. With reference to its well-established circular predecessor, the introduced concept can be regarded as a progression towards a Rowland-sphere spectrometer. Experimental measurements demonstrate a spectral range of 400-1100 nm with a spectral resolution exceeding 102 [λ/Δλ] (Δλ ∼ 6.2 nm @ 633.4 nm). The grating was fabricated using two-photon lithography, enabling the creation of a fine grating structure on a curved surface.